Two cycle engine scavenge ports

ABSTRACT

In a two cycle engine of the type which utilizes the underside of the power piston as a scavenging pump piston and which has the scavenge ports located in the lower end of the cylinder, the provision of a scavenge port or ports of extra height but having normal effective port timing as to opening to the scavenge pump system, additional timing control being effected by means of a port or ports located in the piston side wall.

United States Patent [191 Tenney TWO CYCLE ENGINE SCAVENGE PORTS [76] Inventor: William L. Tenney, Crystal Bay,

Minn. 55323 [22] Filed: Feb. 9, 1972 [21] Appl. No.: 224,756

8/1928 Great Britain 123/73 AA OPTIONAL RESONR NT 0R PULSI TUNED EXHHUJT SYSTEM Mar. 19, 1974 Germany 12 Italy 123/73 AA Primary Examiner-Laurence M. Goodridge Attorney, Westman [5 7] ABSTRACT In a two cycle engine of the type which utilizes the underside of the power piston as a scavenging pump piston and which has the scavenge ports located in the lower end of the cylinder, the provision of a scavenge port or ports of extra height but having normal effective port timing as to opening to the scavenge pump system, additional timing control being effected by means of a port or ports located in the piston side wall. l

15 Claims, 10 Drawing; Figures Agent, or Firm-Dugger, Johnson &

PATENTEUIAR 19 1974 SHEET 1 BF 5 OPTIONAL RESONAUT OR PULSE TUNED EXHHll-ST SYSTG'M PATENTEDMAR 19 W4 3.791.467

swan a o s NU-N k Rh V H z mm l \allr Q A M d kl I! v6 QM mm 3 1 Nu U W v m Q A N w S nu PATENTEDMAR 1 9 I974 sum 3 OF 5 27 .FZZZEZ 4 TWO CYCLE ENGINE SCAVENGE PORTS TERMINOLOGY In describing two cycle engines, terminology can be confusing as regards intake port, inlet port," scavenge port, transfer port, etc. In particular, the terms inlet port and intake port" are used to describe both the power cylinder scavenge ports and the scavenge pump inlet ports. Also, the term transfer port is used to describe both the power cylinder scavenge port and the port at the opposite end of the transfer passageway opening into the scavenge pump system. Confusion isthe result. To reduce such confusion, for the purposes of this patent the'terminology will be employed as follows:

The last port through which the scavenge and charging medium passes on its way into the power cylinder, will be referred to as a scavenge port.

The passageway which connects each scavenge port with the scavenge. pump will be referred to as a transfer passageway.

A port located at the opposite end of each transfer passageway from the scavenge port, which port connects the transfer passageway with the scavenge pump system at the opposite end from the scavenge port, will be referred to as a crankcase outlet port.

Any port which opens from atmosphere into the scavenge pump system to feed or charge the scavenge pump will be referred to as an intake port or inlet port.

Also, the terminology upper dead center, lower dead center," top dead center, bottom dead center, upper or top cylinder or crankcase portion or area, bottom or lower cylinder or crankcase portion, etc. will be used in this patent to refer to an engine so oriented as to have the cylinder longitudinal axis located in a generally vertical plane with the cylinder head uppermost or on top, and the crankshaft and crank chamber located in an area generally directly underneath or at the lower or bottom end of the cylinder.

BACKGROUND OF THE INVENTION The field of the invention is that of the two cycle reciprocating piston type internal combustion engine having the scavenge ports located in that section of the cylinder wall nearest to the bottom dead center position of the power piston, and utilizing the underside of the power piston as a piston for the scavenging pump system.

In prior art engines in the field defined above, the height of the scavenge port opening in the cylinder above the piston timing edge at the bottom dead center position has usually been limited by the requirement for cylinder pressure to blow down through the exhaust port to somewhere near scavenge pump pressure, prior to scavenge port opening. The reason for this height limitation has been that with greater height of scavenge port, the corresponding higher gas pressure in the power cylinder at time of scavenge port opening has caused blow back through the scavenge port into the scavenge pump, with consequent contamination of the incoming fresh charge, interference with the transfer of fresh charge gases into the cylinder, etc., resulting in inefficient engine operation and lesser power output than with lower scavenge port opening. It

should be emphasized, however, that a height of opening of the scavenge port which corresponds to a cylinder pressure somewhat higher than scavenge pump pressure often results in best engine performance. Common practice calls for a scavenge port height varying from 18-20 percent of the piston stroke up to 26-28 percent of the piston stroke, as measured from the piston head timing edge at bottom dead center posi tion, up to the top edge of the scavenge port, although some engines may be found which go beyond these limits.

The height of the scavenge ports described above will be referred to hereafter as the normal scavenge port height. It is that height which is considered by the engine builder to result in best engine operation and is generally limited on the high side by performance limitations on the blow back of high pressure exhaust gases into the piston underside scavenging pump system. On the low side it is generally limited by the aim of providing the largest feasible flow area from the scavenge pump system into the power cylinder, as an aid to high speed running. It might be added that when the upper end portion of the transfer passageway adjacent the scavenge port is angled upward at a steep angle, for instance at 30 from the vertical, the port height may be made somewhat higher than when the upper end portion of the transfer passageway is made more nearly horizontal, or actually horizontal. This is because of the more gradual port opening, with conse quent greater throttling of the blow back gases, in the case of the more steeply upwardly inclined transfer passageway.

When normal height scavenge ports are used, as soon as the piston timing edge uncovers the upper edge of the port, the associated transfer passageway is also open to the scavenge pump system.

Extra height scavenge ports are scavenge ports having their upper edges located at a substantially higher level in the cylinder bore than the upper edges of the normal height scavenge ports, for example, 30 percent of the stroke or higher. Thus they also open earlier than normal height scavenge ports, and unless special provisions are made, will, as described above, result in excessive blow back" into the scavenge pump system with consequent deterioration of engine performance.

Being able to prevent excessive blow back, is of course essential to satisfactory utilization of extra height scavenge ports. In this invention, a port in the piston side wall is used to register with a port in the cylinder side wall forming the lower entrance of the transfer passageway which connects the extra height scavenge port with the piston underside scavenge pump system. It is old in the art to connect a transfer passageway to the scavenge pump via ports in the piston side wall and cylinder wall, but the timing of this port registry has generally been as long or longer than that of the scavenge port at the other end of the passageway. Thus no delaying of the opening of extra height scavenge ports to the scavenge pump is taught. There is no additional blocking of the scavenge pump outlet, in order to prevent harmful blow back from the cylinder scavenge ports into the scavenge pump system. For example, in my US. Pat. No. 3,612,014, in FIG. 6C, a port in a piston wall opens to a transfer passageway but no extra height scavenge port is involved and the timing of the scavenge port opening is the same as the timing of the piston wall port opening. US Pat. Nos. 891,366, and 1,755,260 show engines with ports in the piston skirt. The engines do not have extra height scavenge ports and the piston skirt ports do not provide any additional port timing effect.

ln U.S. Pat. No. 1,729,366, a uniflow type engine is shown. The scavenge ports in this engine are located adjacent the top end of the cylinder with the exhaust port at the lower end of the cylinder, and the scavenge flow is valved into the transfer passageways by ports in the piston skirt. However, the transfer passageways as shown also form portions of the combustion chamber and actually contain spark plugs. The operation of the engine is quite different from the type of engine utilizing the present invention which has the scavenge ports in the lower portion of the cylinder.

In US. Pat. No. 1,109,694 there is shown another uniflow type engine having the scavenge ports located in the upper end of the cylinder, with the opening of the transfer passageway to the scavenge pump system controlled by registry of piston skirt ports with crankcase outlet ports. Here, too, the scavenge ports lie at the top endof the cylinder, not at the bottom end, as in the present invention. With the scavenge ports located at the top end of the cylinder, a substantial portion of the cylinder charge is compressed back into the transfer passageways and trapped therein. The charge trapped in the transfer passageways does not take proper part in the combustion, being sealed off by the upper portion of the piston head at the time combustion is initiated. Obviously this major waste of charge trapped in the transfer passageways is highly undesirable.

The effective scavenge port flow area of prior art engine cylinders, where the scavenge ports are located at the lower end of the cylinder, is limited by the percentage of cylinder wall circumference devoted to scavenge ports, by the entry angles into the cylinder of flow from the scavenge ports, as oriented by the adjacent transfer passageways, and by the normal scavenge port height. The percentage of cylinder wall circumference and also the entry angle of flow is limited by considerations of effective scavenge gas flow pattern within the cylinder, by the portion of cylinder wall required for exhaust or other passageways, and by mechanical considerations such as surfaces required for retention of piston rings, surfaces for piston ring gaps to run on, supports required for strength, etc. Thus the effective scavenge port flow area remains limited in prior art engines.

' It is an object of the invention to overcome the normal height scavenge port limitation and to make possible additional effective scavenge port flow area, with any given scavenge port layout and percentage of cylinder circumference devoted to scavenge ports, by utilization ofextra height scavenge ports. it is also an object of the invention to achieve this additional effective flow area by utilizing, where necessary or desirable, extra height scavenge ports located superposed higher in the cylinder wall above normal height (or less than normal height) scavenge ports.

Power output of prior art engines of the subject type has been limited by insufficient effective cylinder port flow area, and limitation of effective scavenge port flow area constitutes a principal limitation on total effective cylinder port flow area. Accordingly, it is an object of the invention to increase engine power output by increasing effective port flow area by means of utilizing extra height scavenge ports to increase the effective scavenge port flow area.

Extra height scavenge ports can also be used to increase engine power output and efficiency by promoting more efficient or effective scavenge gas flow patterns. For example, the portion of the work cylinder volume located farthest above the scavenge ports is often the most poorly scavenged. By utilizing extra height scavenge ports the distance from this cylinder volume portion to the ports is reduced and scavenging is thus improved. It is likewise desirable under some conditions of design to provide an upward flowing scavenge gas pattern within the cylinder which occupies a larger percentage of cylinder bore cross sectional area, in orderto improve scavenging efficiency. This aim can also be met by use of extra height scavenge ports since they permit increased scavenge port area which can in turn be used to increase the cross sectional area of upward flowing scavenge gases in the cylinder bore. it is thus an object of the invention to increase power output and efficiency by improving the effective scavenge gas flow pattern through use of extra height scavenge ports.

Use of cylinders having high bore to stroke ratios is important from the point of view of permitting higher engine rpm with a cylinder of given piston displacement, without exceeding normal limitations imposed by bearing loads and friction. However, the power output of prior art short stroke engines has often been limited by lack of effective scavenge port flow area in the cylinder. It is an object of the invention to increase the effective scavenge port flow area by use of extra height scavenge ports, in order to permit increased specific power when high bore to stroke ratio cylinders are used. Another limitation on power and efficiency of prior art high bore to stroke ratio cylinders has been the inefficient scavenge gas flow pattern which can result from reduction of scavenge port area in relation to cylinder bore cross sectional area which tends to occur when bore is increased and stroke is decreased. It is thus also an object of the invention to increase power output and efficiency of high bore to stroke ratio cylinders by use of extra height scavenge ports to improve the scavenge gas flow pattern by helping to maintain an upwardly moving'scavenge gas flow path of suitably large cross sectional area in relation to cylinder bore cross sectional area.

High bore to stroke ratio designs generally result in more compact and lighter engines for a given piston displacement volume per cylinder, and it is an object of the invention to render feasible such more compact and lighter engines through the use of extra height scavenge ports which, as above, enable efficient use to be made of high bore to stroke ratio cylinders. Since such engines also develop more horsepower per unit volume of required construction materials, the cost of construction and hence engine cost per horsepower will also tend to decrease. It is thus an object of the invention to provide for reduction of engine cost per horsepower through use of extra height scavenge ports which permit efficient use to be made of high bore to stroke ratio cylinders.

Very high output engines of the subject type tend to develop best power output over only a narrow rpm range or power band. This results in major part from need for very high exhaust ports and subsequent raliance on rpm tuned exhaust systems which utilize pressure waves to offset thelate mechanical closing of these ports. By using extra height scavenge ports, the engine designer can reduce the percentage of cylinder circumference required for adequate scavenge port flow area and thus increase the percentage of circumference left over which can be devoted to exhaust ports. Thus a wider exhaust port can be provided, which will permit use of a lower exhaust port, other things being equal. With a lower exhaust port, performance depends less on an rpm tuned" exhaust system and hence the power band can be widened. In the case of many vehicle applications, widening of the power band is of more importance than increasing the peak power output. It is thus an object of the invention to widen the power band by making possible the use of extra height" scavenge ports.

Other objects of the invention will become apparent as the description proceeds.

, BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a two cycle engine utilizing extra height scavenge ports constructed in accordance with the present invention and taken as on line 1 l in FIG. 7.

FIG. 2 is a vertical sectional view of the cylinder portion of the engine of FIG. 1 with the piston in position wherein the transfer passageways for the extra height scavenge ports are about to be opened to the scavenge pump system;

FIG. 3 is a sectional view taken on the same sight line as FIG. 2 showing the ports in their fully opened condition with the piston at bottom dead center position;

FIG. 4 is a sectional view taken as on line 4-4 in FIG. 7, showing the piston in the same position as in FIG. 1 wherein the scavenge ports are closed;

FIG. 5 is a sectional view taken on the same sight line as FIG. 4 showing the piston in the same position as FIG. 2;

FIG. 6 is a sectional view taken on the same sight line as FIG. 4 showing the piston in the same position as FIG. 3 wherein the ports are fully open with the piston in bottom dead center position;

FIG. 7 is a sectional view taken as on line 7-7 in FIG. 3;

FIG. 8 is a vertical sectional view of a modified form of the engine utilizing extra height scavenge ports made according to the present invention and having a bore to stroke ratio of 1.5:1 and taken as on line 88 in FIG.

FIG. 9 is a sectional view of the form of the invention of FIG. 8 taken as on line 9-9 in FIG. 10; and

FIG. 10 is a sectional view taken as on line l0-10 in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the general construction of a rear compression two cycle engine is illustrated in cross section. The underside of the piston is used as the scavenging pump piston. The two cycle engine illustrated generally at 15 includes a crankcase 16 of usual design and can be made in two sections which are suitably fastened together. The crankcase l6 encloses a crankshaft l7 rotatable on suitable bearings. As shown, the direction of rotation of the crankshaft is counterclockwise. A crank pin 18 on crankshaft 17 is connected to a connecting rod 19. The connecting rod 19 in turn is connected to a piston pin 20 that is mounted in a piston 21. The piston 21 is slidably mounted in the interior cylinder bore 22 in a cylinder 23 attached to the crankcase 16. The cylinder 23 is air cooled as shown and has a number of cooling fins 24 thereon. A cylinder head 25 is attached to the top of the cylinder 23 to close the cylinder bore 22. A spark plug 26 of usual design can be connected to an ignition system to provide ignition of compressed charges in the cylinder bore in a desired manner. The spark plug may be omitted, if compression ignition is utilized, or a glow plug may be used. Fuel injection through the cylinder head (or elsewhere) may also be utilized. The head 25 has cooling fins 27 thereon. The engine could be liquid or otherwise cooled, if desired.

As shown in the first embodiment of the invention the bore diameter to piston stroke ratio is substantially 1:1. Bore to stroke ratios ranging up to l.25:l are within the range which has been generally accepted as best for development of maximum power output per unit volume of piston displacement. Bore to stroke ratios higher than 1.25:1 are generally considered to be high bore to stroke ratios.

An exhaust port 30, which as shown has a height of substantially 50 percent of the piston stroke, opens from the cylinder bore. The exhaust port 30 opens to an exhaust pipe 31 that is attached to the cylinder 23 in a suitable manner. The exhaust. pipe may preferably be connected to a resonant or pulse tuned exhaust sys tem illustrated schematically at 3.2, when the high exhaust port 30 is utilized. As shown in FIG. 7, the exhaust port 30 extends around the periphery of the cylinder bore for a preselected distance. The cylinder wall also has a crankcase inlet port 33 defined therein connected with a suitable tube 34 leading to a carburetor, or other inlet device for introducing a fuel-air or air charge into the scavenge pump system.

The inlet port 33, as shown, is valved by the wall 35 of the piston 21. The bottom edge of wall 35 is used for timing the inlet port. Whenthe piston is at the upper portion of its stroke the inlet port is open to permit introduction of the fuel-air or air charge into the crankcase chamber, which is open to the underside of the piston 2l. This charge is drawn into the crankcase because of the reduced pressure in the crank chamber as,

the piston moves on the upstroke. In the piston position shown in FIG. 1, the inlet port 33 is closed, and a fresh charge which has previously been drawn into the crankcase scavenging pump system by the underside of the piston is being compressed as the piston moves downwardly. The operation of the inlet port 33 valved by a piston skirt is clearly explained and disclosed in my US. Pat. No. 3,612,014 but other types of piston valved inlet ports may also be used. Inlet valving by.

reed valves, rotary valves or other suitable inlet valve devices may likewise be utilized.

Other matters being equal the power output of engines of this type is limited by the engine running speed at which the piston underside scavenge pump system can substantially empty itself at each revolution into the power cylinder via the scavenge ports. Either a higher scavenge pump pressure or larger area scavenge ports and adjoining passageways will permit higher obtainable running speeds and higher power outputs. The circumference of the cylinder bore is a fixed value, and the percentage of its circumference which can be devoted to scavenge port width is limited. The attainment of higher scavenging pressures is limited by normal design and construction requirements in engines employing piston underside scavenging pump systems. Material increases in scavenging pump pressures will result in increased pumping power requirements which will more than offset any resulting increase in gross horsepower developed through higher running speeds. Also, the flow characteristics of the scavenge system are such that the running speed for peak engine rpm will in gen eral only increase as the square root of the scavenge pump pressure.

Enlargement of scavenge port area and corresponding enlargement of passageways and any other ports or restrictions between the scavenge pump system and scavenge ports, has also been tried and has proven effective between the limitations on normal port height (where blow back offsets any gains) and the limitations on port width which result from the need for reserving part of the cylinder wall circumference at the port belt (and directly below) for other requirements such as mechanical strength, retention bars for piston rings, areas for piston ring gaps to ride on, areas for exhaust ports, areas for inlet ports or passageways into the piston underside scavenge pump system, etc. Other limitations upon the scavenge port width result from requirements for developing an efficient, effective and stable scavenge flow pattern within the power cylinder.

When the normal scavenge port height limitation is removed as in this invention, higher running speeds and corresponding higher power outputs can be achieved. The extra height scavenge ports also tend to provide for more effective scavenging because the tops of such ports are closer to the cylinder head. The scavenging medium is thereby introduced to the cylinder bore closer to the cylinder head, which in turn aids in scavenging of the upper portion of the cylinder. Also, extra height scavenge ports may be used by the engine designer to otherwise improve the scavenge gas flow pattern within the cylinder and to widen the power band" or rpm range within which high torque is attained. The extra height scavenge ports to be described thus represent a substantial improvement in the art.

Referring again to FIGS. 1 and 2, diametrically opposite from the exhaust port 30 there is an opposite scavenge port 40. This opposite" scavenge port is an extra height port, extending from an effective lower edge at the level of the top edge 21A of the piston with the piston at bottom dead center position, up to a height of about 45 percent of the stroke above that level.

The height of the upper edge of the extra height scavenge port can be varied from the 45 percent figure as desired. As can be seen, the upper edge 40A of the extra height scavenge port 40 is slightly lower than the level of the upper edge of the exhaust port 30, so that the exhaust port 30 is already opening when the piston is in its position as shown in FIG. 1. However, the timing of the exhaust port opening may be varied considerably without departing from the teaching of this invention.

The extra height scavenge port 40 opens to a transfer passageway 41 that is defined in the cylinder 23, and

- the upper passageway wall portion 42 is positioned so that the acute angle which the wall 42 forms with respect to the longitudinal axis of the cylinder bore 22 is approximately 30. A scavenge gas charge passing through the transfer passageway 41 and out through the extra height scavenge port 40 will therefore be directed upwardly and along the portion of the cylinder bore 22 located opposite exhaust port 30, in order to obtain a desirable scavenge gas flow pattern for scavenging of the cylinder contents. The lower end of the transfer passageway 41 ends at a crankcase outlet port 43 that is valved by a piston wall outlet control port 45. The scavenge gas flow between the scavenge pump system and transfer passageway 41 is valved by the piston wall 35, piston wall port 45 and crankcase outlet port 43 in order to obtain proper timing and prevent excessive blow back.

The transfer passageway 41 is further defined by a cylinder wall section 44 that at its lower end forms the upper edge of crankcase outlet port 43. The surface of wall 44 at its upper portion adjacent port 40 is also angled to help obtain the desired flow pattern for the scavenge gas as it flows through the passageway 41 and out through the scavenge port 40 into the cylinder bore. In the piston position shown in FIG. 1 the crankcase outlet port 43 is blocked off by piston wall 35, thus preventing gas flow through the port.

Also, the length of piston wall as measured from the timing edge 21A of the piston down to the upper edge of piston wall port 45 must be at least equal to the length of the port 40 as measured from its upper edge 40A to the upper edge of wall section 44. This length of piston wall is necessary to prevent blow back of high pressure combustion gases from transfer passageway 41 into the scavenging pump system via piston wall port 45 when the piston moves downwardly so that timing edge 21A uncovers the upper edge 40A of the port 40.

The opening and closing of the transfer passageway 41 to the scavenging pump system is effectively controlled by cooperation between the crankcase outlet port 43 of the passageway 41, and the piston wall outlet control port 45 instead of by piston movement in relation to extra height scavenge port 40. The action of control port 45 in relation to the timing of the extra height opposite scavenge port 40 will be further explained subsequently in this patent.

When a steeply upwardly inclined transfer passageway is used in combination with a normal height scavenge port, flow capacity is reduced below that afforded by the area of the scavenge port itself because of the reduced cross section of the passageway at right angles to the flow at the piston timing edge 21A when the scavenge port and associated passageways are of a similar width. In order to increase the effective flow area an extra height scavenge port may be utilized. When an extra height scavenge port is utilized, the effective minimum cross section at right angles to the flow can be increased to be equal to or greater than that of crankcase outlet port 43, which can be made of a height equal to the height of a normal height scavenge port. Hence, the extra height scavenge port 40 may have flow capacity substantially equal to a normal height scavenge port having an adjacent transfer passageway that is more nearly horizontal, as opposed to being steeply upwardly inclined.

The cylinder wall as shown also contains extra height side" scavenge ports as well as normal height side scavenge ports. The term side scavenge ports as used in this specification refers to those scavenge ports located between opposite scavenge port 40 and the exhaust port 30, andwhich lie in general at the sides of the cylinder wall at each side of exhaust port 30. As can perhaps be best seen by referring to FIGS. 4 and 6, there may be provided extra height side scavenge ports 50, oneach side of the opposite scavenge port 40. These scavenge ports 50 open to transfer passageways 51 that extend through the cylinder block and open, at their lower ends, to crankcase outlet ports 52 that are provided in position so as to be closed by the piston wall 35, when the piston is in position as shown in FIG. 4. The upper edges 50A of the extra height side scavenge ports 50 may be as shown on substantially the same level in the cylinder bore as the upper edge 40A of the extra height opposite scavenge port 40. However, the level of the height of the upper edges of the extra height ports may be varied considerably without departing from the teaching of this invention.

The extra height side scavenge ports 50 are positioned above underlying normal height scavenge ports 55, also on each side of the extraheight opposite scavenge port 40. The end surface 56 of a partition wall member 58 separates the normal height side scavenge port 55 from its associated overlying extra height side scavenge port 50. It should be understood that side scavengeports 55 could. also, if desired, be of less than normal height.

Each of the normal height side scavenge .ports 55 opens to a transfer passageway 57 that is separated from the transfer passageway 51 by the partition wall 58. Each of the transfer passageways 57 leads from a separate crankcase outlet port 59. The ports 59 are positioned above the ports 52, which are the crankcase outlet ports leading to transfer passageways 51 and extra height side scavenge ports 50. The ports 52 and 59 are also separated by the end of the partition wall 58 which forms a divider surface 61.

Additional normal height side scavenge ports 63 can also be provided in the cylinder and these are positioned on adjacent opposite sides of the exhaust port 30, between the exhaust port 30 and the normal height side scavenge ports 55. The scavenge ports 63 open into transfer passageways 64 that are defined in the cylinder block. The transfer passageways 64 open directly to the crankcase which forms the scavenge pump chamber on the underside of the piston 21, or they might also be routedthrough piston skirt ports or windows in known manner. l

The transfer passageway 57, as shown in FIGS. 4, 5 and 6, is also partly defined by a wall 60 forming a part of the wall for the cylinder bore 22, but it should be noted that this wall 60 can be removed if desired, without affecting the effective timing of opening of the scavenge ports 55 or 50 to the scavenge pump system.

The crankcase outlet ports 52 and 59 are designed to communicate with the underside of the piston 21, and therefore with the scavenge pump system, through a piston wall outlet control port 70 provided through the piston wall 35. The control port 70 aligns with its respective crankcase outlet ports 52 and 59 in accordance with movement of the piston, and depending on the position of thepiston during its stroke, the crankcase outlet ports 52 and 59 may be closed off by the piston wall 35, or open to the underside of the piston through the control port 70. It will be noted that when the wall section 60 is omitted, ports 59 and 55 combine to form a single opening without affecting the desired effective control port timing.

As has been discussed in the patent section dealing with the background of the invention, adding height to scavenge ports is detrimental when the port height exceeds a certain proportion of the piston stroke, (corresponding to the normal port height) due to combustion gas blow back into the scavenge pump system. With the additional valving of the present invention as provided by properly timed registry of piston skirt ports with crankcase outlet ports, the normal height limitation is overcome and extra height scavenge ports can be utilized to great advantage.

To follow through a valving cycle, reference will be made simultaneously to FIGS. 1, 2 and 3 which show the valving of the extra height opposite scavenge port 40, and to FIGS. 4, 5 and 6, which show the valving of the extra and normal height side scavenge ports 50 and 55, which are positioned one above the other. In the piston position shown in FIGS. II and 4, the top edge 21A of the piston 21, which is the valving edge, is shown moving downward in the course of the power stroke, and as shown has moved below the upper edge of the exhaust port 30, so that the exhaust gases start to escape. Whereas this is the particular exhaust port timing illustrated, it should be understood that the invention is not necessarily limited to this particular timing of the exhaust port. The valvingedge 21A is then aligned with the upper edge A of the extra height opposite scavenge port 40 and the upper edge A of the extra height side scavenge ports 50. The lower portion of piston wall 35 is blocking off the crankcase outlet port 52 (see FIG. 4). The normal height scavenge ports and 63 are still completely blocked off by upper portions of the piston wall 35.

It is necessary that the section of piston wall 35 extending between timing edge 21A and the upper edge of control port be at least equal to the height of extra height scavenge port 50. This is necessary so that when timing edge 21A uncovers the port edge 50A in the course of the piston descending on the power stroke, blow back into transfer passageway 51 is thereby prevented from entering the scavenge pump system through control port 70, just as the corresponding necessary length of upper piston wall prevents blow back through control port 45, as described previously.

As the piston 21 continues to move downwardly in the cylinder bore 22, from its position as shown in FIGS. 1 and 4, the extra height scavenge ports 40 and 50 will be uncovered. However, the transfer passageways 51 and 41 will continue to be blocked off by the piston wall 35 at the crankcase outlet ports 43 and 52, respectively. Also as described previously, the portions of the piston wall above control ports 45 and 70 prevent blow back through these ports. The exhaust port 30 also continues to be open.

When the piston 21 reaches the position shown in FIGS. 2 and 5, which is the position for opening of normal height scavenge ports, it can be seen that the extra height scavenge ports 40 and 50 in the cylinder wall are uncovered but the crankcase outlet ports 43 and 52 at the lower ends of transfer passageways 41 and 51 are still closed. Thus the scavenge pump system is still sealed off from the cylinder bore 22. During piston movement downwardly the exhaust port 30 has been further opened and the pressure within the cylinder bore 22 therefore has been reduced substantially. The lower timing edge 45A of the piston wall outlet control port 45 in the piston wall 35, (see FIG. 2) is aligned with the lower edge of the wall 44, which forms the upper edge of the crankcase outlet port 43. Likewise,

referring to FIG. 5, the lower timing edge 70A of the piston wall outlet control port 70 in the piston wall 35 is aligned with the upper edge of the crankcase outlet port 52 for passageway 51. In the illustrated position of the piston the pressure within the cylinder bore has blown down to a pressure level where the opening of the scavenge ports to the scavenge pump system may be effected without undue blow back.

Then as the piston 21 continues to move downwardly from its position as shown in FIGS. 2 and 5, the piston wall outlet control port 45 defined in cylinder wall 35 will start to come into registry with the crankcase outlet port 43 leading to transfer passageway 41, opening the port 43. Likewise, the timing edge 70A of the control port 70 will move downwardly, and open the crankcase outlet port 52 leading to transfer passageway 51. At some time shortly after opening of the crankcase outlet control ports, fresh charge gases will start to flow out of the scavenge pump system via the transfer passageways.

In FIGS. 2 and 5, as is usual in the piston position shown, the piston 21 is still blocking off the normal height side scavenge ports 55 and 63. The crankcase outlet port 59 is in communication through the control port 70 with the underside of the piston 21, so that the scavenge pump system is open to the transfer passageway 57, but no flow takes place because of the closing of the port 55 by the upper portion of the piston 21. Likewise, no flow takes place through normal height scavenge port 63, even though transfer passageway 64 is open at its lower end to the scavenge pump system.

When the piston 21 moves downwardly from the position shown in FIGS. 2 and 5 toward the bottom dead center position shown in FIGS. 3 and 6, the transfer passageway 41 is opened through control port 45 to the scavenge pump system, and the transfer passageway 51 is also opened via control port 70 to the scavenge pump system. The normal height ports 63 and 55 are also being uncovered. The extra height ports 40 and were already uncovered, although port 40 was not fully uncovered. Thus all the scavenge ports whether of normal or extra height, are open to the scavenge pump system when the piston moves downwardly from the position shown in FIGS. 2 and 5.

In the bottom dead center position, the crankcase outlet ports 52 and 43 are both fully open, the extra height scavenge ports 50 and 40 are fully open, and the normal height scavenge ports 55 and 63 are also fully open. In this piston position it can readily be seen that the extra height scavenge ports and their associated transfer passageways and crankcase outlet ports greatly increase the total available flow path cross-sectional area for fresh charge flow from the scavenge pump system into the cylinder, as compared to the situation when only normal height scavenge ports are utilized.

After bottom dead center position the piston 21 will start to move upwardly, and the timing edges 45A and 70A of piston wall 35 will start to close off the crankcase outlet ports 52 and 43, while at the same time the timing edge 21A will start to close off the scavenge ports 55 and 63, so that by the time the piston reaches the position shown in FIGS. 2 and 5 on its upstroke, the scavenge pump system is closed off from the cylinder bore 22. Thus, further upward motion of the piston will not have any effect on flow in and out of the scavenge pump system through the transfer passageways. The extra height transfer ports are closed off from the scavenge pump system and so are the normal height scavenge ports.

After the piston has reached its top dead center position, and after compression and ignition have occurred in the usual manner, the piston will again move downwardly, under the urging of the high pressure gases of combustion, and the cycle will repeat. The use of control ports in the piston wall permits increasing the height of the scavenge ports in the cylinder wall without the harmful blow back into the scavenge pump system which would otherwise be caused by too early opening of the scavenge ports during the power stroke; i.e., when the pressure within the cylinder bore 22 is still much higher than the scavenge pump pressure.

The use of a resonant or pulse tuned exhaust system as shown schematically in FIG. 1 can be especially beneficial in combination with the high exhaust port 30 because an exhaust gas pressure wave or pulse can be used to effectively close the exhaust port prior to the actual mechanical closing of the port by the piston, in order to prevent loss of fresh charge through this high port which would otherwise occur.

Referring to FIG. 1, as a modification of the invention, the transfer passageway 41 can be formed so that the surface 42 is not positioned at an acute angle with respect to the longitudinal axis of the bore 22, but rather may be formed so that the scavenge port 40 leads into a passageway wherein the upper wall of the passageway is more nearly at right angles to the longitudinal axis of the cylinder.

The angle of the upper wall of the passageway per se forms no part of the invention. However, in combination with the steeply upwardly inclined upper wall section 42, the use of the single extra height scavenge port 40 together with the piston control port 45 and crankcase outlet port 43 shown, is particularly appropriate for matching of the various available gas flow path cross-sectional areas. When the upper wall section 42 is made horizontal (or even when made somewhere near horizontal), the gas flow path cross-sectional area measured at a right angle to the flow close to extra height port 40, is greatly increased. In such instance the cross-sectional area at this point may become, for example, twice as great as that of the control port 45 and its matching crankcase outlet port 43, but gas flow rate out of the scavenge pump system will increase comparatively little because of the mis-match of flow areas. The cross-sectional areas of ports 43 and 45, plus the cross-sectional area of the transfer passageway 41 in the region indicated at 41A in FIG. 1, will restrict and largely control the gas flow rate regardless of substantially doubling the flow path cross-sectional area of the passageway near scavenge port 40. The outer transfer passageway wall section could be moved further away from wall section 44 in order to increase the crosssectional area of the vertical portion of passageway 41 to match that available adjacent port 40, but the small cross sectional area of ports 42 and 43 would still remain to throttle the flow and thus constitute the principal flow limiting factor. It can thus be seen that when a more nearly horizontal upper passageway section is used to replace the steeply upwardly inclined section 42, gas flow rate out from the scavenge pump is increased little despite great increase in flow path area of the transfer passageway adjacent scavenge port 40.

In order to take advantage of the increased rate of gas flow out of the scavenge pump system ofi'ered by the increased flow path area opened up adjacent to the scavenge port 40 by use of more nearly horizontal upper transfer passage wall sections, it is desirable to utilize a normal height (or less than normal height) scavenge port plus an extra height scavenge port placed above it, as has already been described with regard to ports 55 and 50, with their corresponding transfer passageways and openings into the scavenge pump system. It will be noted that ports 55 and 50, with their horizontal upper transfer passage surfaces, are adequately matched as to flow path area by their accompanying transfer passageways and lower port openings into the scavenge pump system.

Thus the two different arrangements of extra height scavenge port (i. e., the single port arrangement of port 40 and the double port, extra height port placed above normal height port arrangement of ports 50 and 55) are provided as appropriate for flow path area matching according to whether the flow path adjacent the scavenge ports approaches the vertical, or the horizontal. When the flow path approaches the vertical, the single port arrangement is appropriate, as shown (scavenge port 40). When the flow path approaches the horizontal, then the double or superposed port arrangement is appropriate, as shown, (ports 50 and 55) in order to take advantage of the increased flow rate out of the scavenge pump system made possible by the increased square-to'theflow area resulting from the horizontal type flow path at the scavenge ports.

Increased gas flow rate out of the scavenge pump system of course tends toward increased engine power output. In port layouts of the illustrated general type, a steeply upwardly inclined gas flow path is considered beneficial when flowing from scavenge ports located in general on the opposite side of the cylinder wall from the exhaust port. Hence the opposite scavenge port 40 is shown with the adjacent passageway walls steeply upwardly inclined toward the vertical, and with the single extra height port arrangement appropriate for passage and port area matching. Also, for scavenge ports in this type of port layout which are located in the cylinder wall generally at each side of the exhaust port, a scavenge gas flow approaching the horizontal is often considered best. Thus for the extra height side scavenge ports a superposed or double type layout is shown in order to match the increased flow path area afforded by the horizontal type flowpath.

It should be understood that the engine designer may not wish in all instances to utilize the increased flow rate out of the scavenge pump made possible by the increase flow path area afforded by the more horizontal type scavenge port flow path. For example, the flow rate out of the scavenge pump might be adequate for the desired engine revolution rate, without any increase in flow rate out of the scavenge pump system. At the same time a lower velocity, more nearly horizontal and of greater cross sectional area scavenge gas flow into cylinder bore 22 might be desired. In such instance the horizontal type upper passageway wall 42A would be utilized as illustrated in combination with the single extra height port 40 and control port 45. Also it should be understood that where more steeply upwardly inclined gas flow out of the scavenge ports is desired, the single extra height port arrangement of 40 with accompanying single type transfer passageway 41 plus attendant single crankcase outlet port 43 is not the only extra height port arrangement available to the designer.

In some cases it may be desirable to utilize the double or superposed arrangement of ports 55 and 50 in combination with varying degrees of transfer passageway guided inclination of gas flow toward the vertical.

In summary, the valving by the piston wall is such that the timing edges 70A and 45A of the control ports in the piston wall do not open the transfer passageways for the extra height scavenge ports until the piston has moved down to a position wherein normal height scavenge ports would be opening. Thus by employing secondary valving means, time of opening of the extra height ports to the scavenge pump system is controlled to be substantially the same number of degreesof rotation of the crank as with normal height scavenge ports. However, the total flow path area from the scavenge pump system to the cylinder bore is greatly increased.

In FIG. 7, a cross section of the cylinder is shown. It can be seen in this figure that the side scavenge ports and their adjoining transfer passageway wall surfaces are directed across the piston head toward the cylinder bore area located on the opposite side from exhaust port in order to obtain the desired scavenge gas flow pattern within the cylinder. The steeply upwardly inclined wall section 42 of the transfer passageway 41 aids in obtaining the desired scavenge flow pattern by directing the flow upwardly. Thus the scavenging gases flow generally upwardly in the portions of the cylinder bore area on the opposite side of the bore from the ex haust port, then across the cylinder head and finally flow back down along the wall of the cylinder above the exhaust port while scavenging spent combustion gases out through the exhaust port.

The additional useful effective flowpath area resulting from use of extra height scavenge ports can also be clearly seen in FIG. 7. The two additional transfer passageways 51 and the twice usual depth transfer passageway 41 (as measured on a radialbisecting line), are made feasible by use of extra height scavenge ports.

While the particular port layout shown in FIGS. 1-7 is a desirable one since it results in an efficient scavenge gas flow pattern within the cylinder bore and at the same time makes substantially the entire cylinder bore circumference at the port belt area available for ports, it should be understood that the invention is by no means limited to this layout. Extra height scavenge ports with their accompanying piston wall control ports may also be utilized to advantage in combination with other port layouts. For example, opposite extra height port may be omitted and scavenge pump inlet port 33 may be relocated on the side of the cylinder left vacant by omitting port 40. This is the more usual loop scavenging arrangement. There are also available many forms of cross scavenging port layouts in which the scavenge ports are located principally opposite or across from the exhaust port. In the cross scavenging arrangements various types of flow directing baffles on the piston head have often been used, and such baffles have also been used as part of loop scavenging" arrangements. There are also other types of scavenge port layouts, piston head arrangements, exhaust port arrangements, etc. too numerous to describe, which are used in the subject type engines having the scavenge ports located at the lower end of the cylinder, adjacent the piston head at the bottom dead center position. It is to be understood that the extra height scavenge ports plus piston wall control ports of i this invention can be utilized as desired in combination with any of these port layouts, without departing from the teaching of the invention.

In FIGS. 8, 9 and 10, a modified form of the invention is illustrated. In this particular form of the invention, the bore to stroke ratio is illustrated as being 1.5:]. The present invention utilizing extra height scavenge ports has particular adaptability to engines with such a highly over square bore to stroke ratio.

In present engines having bore to stroke ratios greater than the 1:1 to 1.25:1 range, the engines generally deliver no more power than their 1:1 to 1.25:1 bore to stroke ratio counterparts, and quite often deliver less. A principal reason for this is the fact that the gas flow through the cylinder scavenge port is largely limited by the effective scavenge port flow area, at any given scavenge pump pressure, and in high bore to stroke ratio cylinders the effective scavenge port flow area is less than in 1:1 to 1.25:1 bore to stroke ratio cylinders having the same port layout and timing, the same piston displacement. This results because with increasing cylinder bore diameter the piston displacement increases as the square of the bore diameter but the cylinder wall area available for ports increases only in direct ratio with bore diameter. In most instances the rpm for maximum power of a high bore to stroke ratio engine of a given single cylinder displacement having a given port layout and timing will not exceed that of a 1:1 to l:25:l bore to stroke ratio range cylinder of the same piston displacement, same port layout as to percentage of cylinder wall diameter devoted-to ports, and same timing. Hence the indicated power of the high bore to stroke ratio cylinder will tend to be no greater, as has been proved in actual practice.

By use of the extra height scavenge ports of this invention, however, scavenge port area can be increased so that the .effective port flow area through a high bore to stroke ratio cylinder of a given piston displacement, port layout and port timing, can be made sufficient to meet the gas flow demands of the increased rpm permitted by the generally accepted 4,000 feet per minute piston speed limit in combination with the short piston stroke. Thus with a bore to stroke ratio in the 2:1 range, the rpm and the power output potential is 1.6 times greater than with a bore to stroke ratio in the 1:1 range. A 60 percent increase in power potential in combination with the lesser weight and cost inherent in the short stroke arrangement, is thus made feasible in a very high bore to stroke ratio cylinder of a given displacement through use of extra height scavenge ports.

Another factor working against the high bore to stroke ratio cylinders when only normal height scavenge ports are utilized, is based in the ratio of the crosssectional area of the upward flowing scavenge gas stream in the cylinder, to the cylinder bore crosssectional area. If the cross-sectional area of the upward flowing gas stream is correct in relation to cylinder bore cross-sectional area so that the resulting scavenge flow pattern gives best scavenging efficiency with the usual 1:1 to 1.25:1 bore to stroke ratio range cylinder, then the same port layout with regard to percentage of cylinder wall port widths, timing, etc. will tend to result in a smaller and presumably less favorable ratio of upward flowing scavenge gas stream to cylinder bore cross sectional area, in a cylinder of high bore to stroke ratio. However, when the extra height scavenge ports of this invention are used, the. scavenge port efiective flow area can thereby be increased sufficiently to maintain the desired ratio of upward flowing scavenge gas cross sectional area to cylinder bore cross sectional area, even in cylinders of very high bore to stroke ratios.

Referring specifically to FIGS. 8 and 9, the lower portions of a crankcase are shown only fragmentarily, and some of the operational details have been omitted. In FIG. 9, a two cycle engine is shown with a piston 94 which includes a piston pin 92 mounted in suitable support bosses 93. The piston pin 92 passes through the small end eye of connecting rod 91 in the usual manner. Piston pin 92 is mounted near the bottom of the piston wall 95, in order to provide space for the location of piston wall ports above the pin. Other known means may be used to locate the piston pin so as not to interfere with the piston wall ports. For example, with the pin placed higher in the piston a shorter pin could be used so that its extremities would not interfere with the piston wall ports. A narrow connecting rod and eye could be used in combination with the short piston pin, and the pin support bosses in the piston could be designed to cooperate with the ports without interfering with them.

The engine includes a cylinder 96 mounted on the crankcase. The piston 94 is mounted for movement in cylinder bore 97. The cylinder has a cylinder head 98 mounted thereon. Suitable cooling fins 99 are provided on the cylinder and fins 100 are provided on the head 98 for adequate cooling. A spark plug can be utilized if desired, or the engine can be a compression ignition or glow plug engine. Fuel injection through the cylinder head or elsewhere may also be utilized.

The cylinder bore has an exhaust port 103 therein leading to an exhaust pipe 103A that may be part of a resonant or pulse tuned exhaust system as previously described. As shown in this form of the invention, the exhaust port 103 may have a height of substantially 50 percent of the stroke, and may extend around the periphery of the cylinder bore as shown in FIG. 10. It should be understood, however, that the exhaust port dimensions may be varied widely without departing from the teaching of this invention.

An opposite extra height scavenge port 104 opens to the cylinder bore 97 on the opposite side thereof from the exhaust port, and this extra height scavenge port 104 opens from a transfer passageway 105 that has a steeply upwardly slanted wall 106 which is in this instance positioned at an angle of approximately 30 with the respect to the longitudinal axis of the cylinder bore 97. The extra height scavenge port 104 has an upper edge that is located at a level located substantially 45 percent of the stroke above bottom dead center position of the piston upper timing edge 94A. Again, the exact timing of the extra height port maybe varied. The transfer passageway 105 leads from a crankcase outlet port 107 that communicates through a piston wall outlet control port 108 in the piston wall 95, which controls the scavenge pump outlet.

The control port 108 has a timing edge that does not uncover the crankcase outlet port 107 until the piston has moved on its downstroke to a position where the combustion gas pressure inside the bore 97 has blown down through the exhaust port 103 to somewhere near the gas pressure within the scavenge pump system.

In this form of the invention, having high bore to stroke ratio and consequent desirability for obtaining matching high scavenge port flow areas, the side scavenge ports include upper or extra height scavenge ports and underlying normal height scavenge ports in the walls of the cylinder bore extending substantially all the way between the exhaust port and the opposite extra height scavenge port. For example, the side scavenge ports next to the opposite scavenge port 104 comprise an extra height scavenge port 110 on each side of the oppositescavenge port 104. A typical showing of the transfer passageway for the extra height scavenge ports 110 appears in FIG. 9 wherein a transfer passageway 111 extends from a crankcase outlet'port 112 to the extra height scavenge port 110. Underlying each of the extra height scavenge ports 110 is a normal height scavenge port 113 that opens into a transfer passageway 114, which in turn communicates with a crankcase outlet port 115 at the lower end of the transfer passageway 114. A cylinder wall portion 116 may be provided, if desired, between the lower edge of the normal height scavenge port 113 and the upper edge of the crankcase outlet port 115 leading to the transfer passageway 114. The transfer passageways 1 l4 and 1 1 l are separated by a wall member 117, the ends of which form a divider bar between the scavenge ports 110 and .113, and also a divider bar between the ports 112 and 115. Communication between the transfer passageways 1 11 and 1 14 and the scavenge pump system is made through a piston wall outlet control port 120 in the side wall 95 of the piston 94. The bottom timingedge 120A of the control port 120 controls the opening of the extra height scavenge port 110 with respect to the scavenge pump chamber on the underside of the piston 94. The piston upper timing or valving edge 94A will move downwardly past the upper edge of the extra height scavenge port on the power stroke, but the piston wall 95 will still block off the crankcase outlet port 112 so that passageway 111 remains closed to the scavenge pump system until the timing edge 120A moves below the upper edge of the port 112.

A second set of extra height and normal height side scavenge ports are positioned between the ports 110 and 113, and the exhaust port 103. The second set includes a pair of extra height scavenge ports 125 (one on each side of the cylinder) each of which opens through a passageway 126 to a crankcase outlet port 127 (see FIG. 8) in the lower portion ofthe wall of the cylinder bore 97. The crankcase outlet port 127 is aligned with and valved by a piston wall outlet port corresponding to the port 108 for valving port 107.

The normal height scavenge ports 130 which underlie the extra height scavenge ports 125 open to passageways 131 defined in the cylinder block. The passageways 131 are not valved by the piston wall, but open directly into the scavenge pump system chamber 132. The passageways 131 are. formed so that the scavenge gas flow area is substantially the same as the area of ports 130, and the valving of the normal height ports 130 is done by the upper edge'94A of the piston. The location of passageways 13] illustrates one of several possible transfer passageway configurations usable for the normal height scavenge ports.

The operation of the engine illustrated in FIGS. 8, 9 and is the same as previously explained, except that in this instance there are four extra height side scavenge ports as well as the extraheight opposite scavenge port 104. There are also four nonnal height side scavenge ports 113 and 130 which are valved by the piston timing edge 94A. The opening of the transfer passageways for the normal height scavenge ports 113 to the scavenge pump system is through the piston wall outlet port 120 in the piston wall 95. The opening of the trans- 5 fer passageways 111 to the scavenge pump system occurs on the power stroke when timing edge 120A uncovers crankcase outlet ports 112.The transfer passageway 126 is opened to the scavenge pump system when the provided port in the piston wall 95 uncovers the crankcase outlet port 127. Passageway 105 opens to the scavenge pump system when port 108 uncovers crankcase outlet port 107. In bottom dead center position all scavenge ports are open to the scavenge pump system.

On the upstroke the piston wall operates to close off the transfer passageways 105, 111 and 126 for the extra height scavenge ports at the same piston position as when the timing edge 94A is closing off the normal height scavenge ports.

The piston wall lengths in relation to the position of the piston wall control ports necessary to prevent blow back through the control ports when the upper portion of the piston is not blocking the extra height scavenge ports is the same as that discussed in connection with the first form of the invention. Also, the walls of the transfer passageways leading to each of the scavenge ports are angled at the proper relationship to the cylinder bore axis in order to obtain the desired scavenge flow pattern. However, the invention is not limited to any particular angular positioning of the transfer passageway walls.

In addition to increased power output the added benefits of reduced weight, size and cost for the same cylinder displacement are possible with high bore to stroke ratios when extra height scavenge ports are used. However, with only normal height scavenge ports increased power output is not the usual result.

The previous discussion has dealt primarily with the increase of scavenge port and transfer passageway flow areas. Effective port flow area through the cylinder bore is also controlled by exhaust port area. With the great increases in scavenge port. area available from any given percentage of cylinder wall circumference when extra height scavenge ports are used, it becomes within the engine designers power to increase the percentage of cylinder wall area devoted to exhaust ports. The designer can thus increase the exhaust port area, while at the same time correspondingly reducing the percentage of cylinder wall circumference reserved for the scavenge ports, and still increase the scavenge port area. Therefore with the aid of the extra height scavenge ports both the exhaust port and scavenge port area can be increased, if desired, in order to increase total effective port flow area.

The extra height scavenge ports can also be used to widen the power band; i.e., to increase the rpm range through which high torque and high power is produced. With very high exhaust ports in combination with the usual expansion chamber or resonant type exhaust system, very high torque and horsepower output is available over a comparatively narrow rpm range. In many vehicle and other applications, however, a broader rpm range is desirable. By taking advantage of the design possibilities afforded by the extra height scavenge ports, the effective cylinder port flow area can be increased even while using lower height exhaust ports. With the lower height exhaust ports, power and torque become less dependent upon arrival of a positive exhaust gas pressure wave at a crucial point in the cycle, and power and torque can be maintained at a high level over a broader rpm range because of less need for reliance on an rpm tuned" exhaust system to cause earlier effective exhaust port closing.

In summary, extra height scavenge ports can be used to increase the effective cylinder port flow area without upsetting the desired effective, efficient and stable scavenge flow pattern, and when this is done the running speed of the engine at maximum power output can also be increased. The power output at the higher running speed will also increase. Also, the power output can be increased even more when the rpm limitation for mechanical reasons, caused by the presently accepted approximately 4,000 feet per minute limiting piston speed range, is raised by utilizing cylinders of high bore to stroke ratio. At the same time, engine weight, size and cost, (due to requiring less material for construction) are reduced.

Extra height scavenge ports can also be used to increase engine power output and at the same time to increase engine efficiency through generation of more effective scavenge gas flow patterns.

Finally, extra height scavenge ports can be used to broaden the power band; i.e., the rpm range over which high torque and power are produced.

What is claimed is:

1. In a two cycle internal combustion engine having a piston with a head, a skirt portion, and an underside opposite the head end, said engine being of a type which has a chamber open to the underside of the piston comprising a scavenge pump system, a power cylinder defined by a wall and having a first end and a distal end, said piston being slidably mounted in said cylinder for reciprocation in said cylinder through power and compression piston strokes between a bottom dead center position adjacent said first end and a distal position, said piston having a piston head timing edge, exhaust means opening to relieve combustion pressure in said cylinder during part of each power stroke of said piston, scavenge port means opening through the wall of said cylinder, transfer passageway means connected to said scavenge portmeans and communicating with said scavenge pump system, said scavenge port means in said cylinder being positioned to have the major portion thereof positioned closer to the first end of said cylinder than to said distal end of said cylinder, said scavenge port means having distal portions positioned remote from said first end, said distal portions being covered and uncovered by said piston timing edge as said piston reciprocates in said cylinder, separate valving means timed by piston position to control communication between said transfer passageway means and said scavenge pump system to prevent such communication until the exhaust nieans has opened during each I power stroke and until the piston head timing edge is closer to its bottom dead center position than the distal edges of said scavenge port means by a distance which uncovers an area of said scavenge port means effective to increase power output of said engine by increasing mass flow rate from said scavenge pump system, said separate valving means comprising a scavenge pump system outlet port opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt positioned to align with said outlet port to control communication between said transfer passageway means for said scavenge port means and said scavenge pump system through said outlet port.

2. The combination as specified in claim 1 wherein said scavenge port means extends from the distal edge thereof toward the first end of said cylinder substantially to a level corresponding to the bottom dead center position of the timing edge of said piston.

3. The combination as specified in claim 1 wherein said scavenge port means includes first scavenge port means no greater than normal height and second extra height scavenge port means, said first scavenge port means opening to separate transfer passageway means from the transfer passageway means for second scavenge port means, said first scavenge port means being positioned between said second scavenge port means and said first end of the cylinder, communication between the scavenge pump and the cylinder through said first scavenge port means being controlled by the piston and the piston head timing edge, and said separate valving means being associated with the second scavenge port means.

4. The combination as specified in claim 1 wherein said scavenge port means comprises a continuous scavenge port extending from substantially adjacent a reference line defined by the piston timing edge in the piston bottom dead center position, to the distal edge of said continuous scavenge port, and said distal edge being positioned more than 30 percent of the piston stroke from said reference line.

5. A two cycle engine having a cylinder and a power piston with a piston skirt, a piston head timing edge and an underside, and which utilizes the underside of the piston as a piston of a scavenge pump system and which has scavenge ports located in a first end portion of the cylinder, said cylinder having said first end adjacent a line along the piston timing edge with the piston in a bottom dead center position, and said cylinder having a distal end, said cylinder having exhaust means which opens to discharge exhaust gases, transfer passageway means leading from said scavenge pump system to the scavenge ports, said scavenge ports in said cylinder being covered and uncovered by said piston timing edge as the piston reciprocates in the cylinder, said scavenge ports in said cylinder having distal edges spaced from the first end of said cylinder, said scavenge ports extending toward the first end from the distal edges, the major portion of said scavenge ports being closer to said first end than to said second end, at least one of said scavenge ports having portions of significant flow area adjacent said distal edges which are uncovered by said piston when the piston timing edge is closer to the first end of the cylinder than substantially 20 percent of the piston stroke, separate valvingmeans to control flow from said scavenge pump system into said transfer passageway means and then to said at least one scavenge port independent of said piston head timing edge to prevent communication from said scavenge pump system to said at least one scavenge port until the piston has reached a preselected position wherein a portion of said at least one scavenge port having an area for flow from said scavenge pump system which iseffective to increase power output of said engine is uncovered and said exhaust means is open, said separate valving means comprising a scavenge pump system outlet port opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control ,port defined in said piston skirt positioned to align with said outlet port at said preselected position to control communication between said transfer passageway means and its associated scavenge port and said scavenge pump system.

6. In a two cycle engine having a cylinder and a power piston which has a skirt and a piston head timing edge, and which utilizes the power piston as a piston of a scavenge pump, said cylinder being defined by a cylindrical wall having an inner surface, scavenge port means defined through said wall and open to said cylinder, the cylinder having a support wall portion and a power chamber portion, said scavenge port means opening to said power chamber portion, said scavenge port means opening to said power chamber portion, exhaust port means opening to said power chamber portion, said power chamber portion having a first end portion adjacent a reference line along the piston timing edge with the piston in a piston bottom dead center position, and a distal end, at least a major portion of said scavenge port means being located in the first end portion of said power chamber portion of said cylinder and occupying a major portion of the cylinder wall circumference not occupied by said exhaust port means, transfer passageway means leading from said scavenge pump to the scavenge port means, said scavenge port means being covered and uncovered by said piston as the piston reciprocates in the cylinder, said scavenge port means having a distal edge spaced from said reference line, at least some of said scavenge port means being uncovered a substantial amount when the piston timing edge is closer to the first end of said power chamber portion of said cylinder than substantially percent of its stroke, said transfer passageway means opening to a scavenge pump outlet control port defined in the support portion of said cylinder wall, said outlet control port aligning with the skirt of said piston when said piston first uncovers said some scavenge port means when the piston is moving toward its bottom dead center position from the distal end, and a piston skirt control port defined in said piston skirt and positioned in alignment with said outlet port only when said piston is closer to its bottom dead centerposition than a preselected position wherein a substantial portion of said some scavenge port means and a portion of said exhaust port means is uncovered.

7. The two cycle engine of claim 6 wherein said some scavenge port means terminates at a position spaced toward the distal endrof said power chamber portion from said reference line, and a second scavenge port is positioned between said some scavenge port means and the reference line, said second scavenge port opening to a separate transfer passageway communicating with said scavenge pump whenever the piston uncovers said second scavenge port.

8. In a two cycle engine having a piston with a piston skirt and a head and a rear side and being of the type which has a chamber comprising a scavenge pump system open to the rear side of the piston, a power cylinder having abore defined by a cylinder wall and having a first end and a second distal end, said cylinder wall having a lower support portion, a piston slidably mounted in said cylinder for reciprocation through a piston stroke in said cylinder, said piston having a piston head timing edge movable through said piston stroke from a bottom dead center position adjacent a first end of said cylinder toward the second distal end of said cylinder, said cylinder havinga diameter that is greater than 1.25 times the piston stroke, scavenge port means defined through the wall of said cylinder and communicating with said bore, exhaust port means defined through a wall of said cylinder and positioned ad jacent the first end of said cylinder and extending toward the distal end a preselected amount, transfer passageway means connected to said scavenge port meansand communicating with said scavenge pump system, said scavenge port means being positioned around the periphery of said cylinder wall to occupy a substantial portion of the cylinder wall, said scavenge port means in said cylinder positioned to have major portions thereof positioned closer to the first end of said cylinder than to said distal end of said cylinder, at least one of said scavenge port means having a distal portion, said distal portion being covered and uncovered by said piston as it reciprocates in said cylinder, valving means remote from said one scavenge port means to control communication between a transfer passageway connected to said one scavenge port means and said scavenge pump system to prevent communication between the scavenge port system and the cylinder through said one scavenge port means until the piston timing edge is substantially closer to its bottom dead center position than the distal edge of said one scavenge port means, and at least a portion of said exhaust port means is uncovered to permit pressure in said cylinder to reduce toward atmospheric pressure, said valving means comprising a scavenge pump system outlet port defined in the lower support portion of said cylinder wall and opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt open to the rear side of the piston and positioned to align with said outlet port during portions of the piston stroke.

9. The combination as specified in claim 8 wherein said cylinder has a lower support portion, and said piston has a piston skirt supported by said lower portion during portions of the piston stroke and wherein said valving means comprises a scavenge pump system outlet port defined in the lower support portion and open ing to said transfer passageway means and to said Scav enge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt open to the rear side of the piston and positioned to align with said outlet port during portions of the piston stroke.

10. The combination as specified in claim 8 wherein said distal edge of said scavenge port means extends at least 30 percent of the piston stroke from a reference line defined by the piston timing edge in bottom dead center position.

11. The combination as specified in claim 8 wherein said one scavenge port means extends continuously from the distal edge thereof toward the first end of said cylinder substantially to a level corresponding to the bottom dead center position of the timing edge of said piston.

12. The combination as specified in claim 8 wherein said scavenge port means includes first scavenge port means no greater than normal height and second extra height scavenge port means, said first scavenge port means opening to transfer passageways separate from the transfer passageway means for second extra height scavenge port means, said first scavenge port means being positioned between said second scavenge port means and said first end of the cylinder, and said valving means opening the transfer passageway means associated with the second scavenge port means to communication with the scavenge pump system during substantially the same portion of piston stroke as when the piston timing edge uncovers said first scavenge port means.

13. In a two cycle engine having a cylinder and a power piston which has a skirt and a piston head timing edge, and which utilizes the underside of the power piston as a piston of a scavenge pump, said cylinder being defined by a cylindrical wall having an inner surface, scavenge port means defined through said wall and open to said cylinder, the cylinder having a lower wall portion and an upper portion, said scavenge port means opening to said upper portion, said upper portion having a first end adjacent a reference line along the piston timing edge with the piston in bottom dead center position and having a distal end, at least the major portion of said scavenge port means being located adjacent the first end of said upper portion of said cylinder, transfer passageway means leading from said scavenge pump to the scavenge port means, said scavenge port means being covered and uncovered by said piston as the piston reciprocates in the cylinder, said scavenge port means having a distal edge spaced from said reference line by a distance such that portions of said scavenge port means are uncovered by the piston timing edge prior to the time the pressure of combustion gases in the upper portion of the cylinder has reduced to nearly rear compression pressure, said transfer passageway means opening to a scavenge pump outlet control port defined in the lower portion of said cylinder wall, said outlet control port aligning with and being blocked by the skirt of said piston when the piston is moving toward its bottom dead center position from the distal end to prevent amounts of said combustion gases harmful to engine performance from flowing back into the scavenge pump system, and a piston skirt control defined in said piston skirt and positioned in alignment with said outlet port only when the pressure of combustion gases has reduced to a desired level.

14. In a two cycle engine having a piston with a piston skirt, a head end and an underside opposite the head end, said engine being of'a type which has a chamber open to the underside of the piston comprising a scavenge pump system, a power cylinder defined by a wall and having a first end and a distal end, said piston being slidably mounted in said cylinder for reciprocation in said cylinder during a piston stroke from a bottom dead center position adjacent said first end to a distal position, said piston having a piston timing edge, an exhaust port defined in said cylinder on one side thereof, an opposite scavenge port means extending from a line generally along the piston head timing edge in bottom dead center position to a distal end at a level more than 30 percent of the piston stroke toward the distal end of the cylinder, with the major portion of said opposite scavenge port means being closer to the first end of said cylinder than the distal end, said opposite scavenge port means being positioned generally diametrically opposite the center of said exhaust port, a first transfer passageway connected to said opposite scavenge port means, side scavenge port means in the cylinder wall located circumferentially between the opposite scavenge port means and the exhaust port and being positioned closely adjacent the sides of the opposite scav enge port means and occupying a substantial portion of the cylinder wall circumference between the side edges of the opposite scavenge port means and the side edges of the exhaust port, second transfer passageway means opening to the scavenge pump system from said side scavenge port means, said scavenge port means being covered and uncovered by said piston timing edge as said piston reciprocates in said cylinder, and separate valving means timed by piston position to control communication between said first transfer passageway and said scavenge pump system to prevent such communication until the piston timing edge is closer to its bottom dead center position than the distal edge of said opposite scavenge port means, said separate valving means comprising a separate scavenge pump system outlet port opening to said first transfer passageway and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke when portions of the opposite scavenge port means of significant flow area and at least a portion of said exhaust port are uncovered by the piston, and a separate control port, corresponding to the outlet port, defined in said piston skirt and positioned to align with said outlet port to control communication between said first transfer passageway and said scavenge pump system through said outlet port.

15. The combination as specified in claim 14 wherein said side scavenge port means includes at least two normal height scavenge port means, one on each side of said opposite scavenge port, at least two separate extra height scavenge port means, one on each side of said opposite scavenge port, said extra height scavenge port means being positioned toward the distal end of the cylinder from the normal height scavenge port means, said extra height scavenge port means having separate transfer passageways connected thereto, and piston skirt port valving means associated with the transfer passageways for the extra height port means to control communication between said scavenge pump system and the extra height scavenge port means, portions of said extra height scavenge port means being uncovered before the valving means associated therewith opens the extra height port means to the scavenge pump system.

CERTIFICATE OF CORRECTION Patent No. 3,797,467 Dated March 19. 1974 I Inventor-(s) William L. Tenney It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

F 7 Column 22, line 15 (Claim 8, line 25), after "cylinder" insert "being Signed and sealed this 13th day of August 1974.

j (S EAL) Attest: V

MCCOY M. GIBSON, JR. (3. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. In a two cycle internal combustion engine having a piston with a head, a skirt portion, and an underside opposite the head end, said engine being of a type which has a chamber open to the underside of the piston comprising a scavenge pump system, a power cylinder defined by a wall and having a first end and a distal end, said piston being slidably mounted in said cylinder for reciprocation in said cylinder through power and compression piston strokes between a bottom dead center position adjacent said first end and a distal position, said piston having a piston head timing edge, exhaust means opening to relieve combustion pressure in said cylinder during part of each power stroke of said piston, scavenge port means opening through the wall of said cylinder, transfer passageway means connected to said scavenge port means and communicating with said scavenge pump system, said scavenge port means in said cylinder being positioned to have the major portion thereof positioned closer to the first end of said cylinder than to said distal end of said cylinder, said scavenge port means having distal portions positioned remote from said first end, said distal portions being covered and uncovered by said piston timing edge as said piston reciprocates in said cylinder, separate valving means timed by piston position to control communication between said transfer passageway means and said scavenge pump system to prevent such communication until the exhaust means has opened during each power stroke and until the piston head timing edge is closer to its bottom dead center position than the distal edges of said scavenge port means by a distance which uncovers an area of said scavenge port means effective to increase power output of said engine by increasing mass flow rate from said scavenge pump system, said separate valving means comprising a scavenge pump system outlet port opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt positioned to align with said outlet port to control communication between said transfer passageway means for said scavenge port means and said scavenge pump system throuGh said outlet port.
 2. The combination as specified in claim 1 wherein said scavenge port means extends from the distal edge thereof toward the first end of said cylinder substantially to a level corresponding to the bottom dead center position of the timing edge of said piston.
 3. The combination as specified in claim 1 wherein said scavenge port means includes first scavenge port means no greater than normal height and second extra height scavenge port means, said first scavenge port means opening to separate transfer passageway means from the transfer passageway means for second scavenge port means, said first scavenge port means being positioned between said second scavenge port means and said first end of the cylinder, communication between the scavenge pump and the cylinder through said first scavenge port means being controlled by the piston and the piston head timing edge, and said separate valving means being associated with the second scavenge port means.
 4. The combination as specified in claim 1 wherein said scavenge port means comprises a continuous scavenge port extending from substantially adjacent a reference line defined by the piston timing edge in the piston bottom dead center position, to the distal edge of said continuous scavenge port, and said distal edge being positioned more than 30 percent of the piston stroke from said reference line.
 5. A two cycle engine having a cylinder and a power piston with a piston skirt, a piston head timing edge and an underside, and which utilizes the underside of the piston as a piston of a scavenge pump system and which has scavenge ports located in a first end portion of the cylinder, said cylinder having said first end adjacent a line along the piston timing edge with the piston in a bottom dead center position, and said cylinder having a distal end, said cylinder having exhaust means which opens to discharge exhaust gases, transfer passageway means leading from said scavenge pump system to the scavenge ports, said scavenge ports in said cylinder being covered and uncovered by said piston timing edge as the piston reciprocates in the cylinder, said scavenge ports in said cylinder having distal edges spaced from the first end of said cylinder, said scavenge ports extending toward the first end from the distal edges, the major portion of said scavenge ports being closer to said first end than to said second end, at least one of said scavenge ports having portions of significant flow area adjacent said distal edges which are uncovered by said piston when the piston timing edge is closer to the first end of the cylinder than substantially 20 percent of the piston stroke, separate valving means to control flow from said scavenge pump system into said transfer passageway means and then to said at least one scavenge port independent of said piston head timing edge to prevent communication from said scavenge pump system to said at least one scavenge port until the piston has reached a preselected position wherein a portion of said at least one scavenge port having an area for flow from said scavenge pump system which is effective to increase power output of said engine is uncovered and said exhaust means is open, said separate valving means comprising a scavenge pump system outlet port opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt positioned to align with said outlet port at said preselected position to control communication between said transfer passageway means and its associated scavenge port and said scavenge pump system.
 6. In a two cycle engine having a cylinder and a power piston which has a skirt and a piston head timing edge, and which utilizes the power piston as a piston of a scavenge pump, said cylinder being defined by a cylindrical wall having an inner surface, scavenge port means defined through said wall and open to said cylinder, The cylinder having a support wall portion and a power chamber portion, said scavenge port means opening to said power chamber portion, said scavenge port means opening to said power chamber portion, exhaust port means opening to said power chamber portion, said power chamber portion having a first end portion adjacent a reference line along the piston timing edge with the piston in a piston bottom dead center position, and a distal end, at least a major portion of said scavenge port means being located in the first end portion of said power chamber portion of said cylinder and occupying a major portion of the cylinder wall circumference not occupied by said exhaust port means, transfer passageway means leading from said scavenge pump to the scavenge port means, said scavenge port means being covered and uncovered by said piston as the piston reciprocates in the cylinder, said scavenge port means having a distal edge spaced from said reference line, at least some of said scavenge port means being uncovered a substantial amount when the piston timing edge is closer to the first end of said power chamber portion of said cylinder than substantially 20 percent of its stroke, said transfer passageway means opening to a scavenge pump outlet control port defined in the support portion of said cylinder wall, said outlet control port aligning with the skirt of said piston when said piston first uncovers said some scavenge port means when the piston is moving toward its bottom dead center position from the distal end, and a piston skirt control port defined in said piston skirt and positioned in alignment with said outlet port only when said piston is closer to its bottom dead center position than a preselected position wherein a substantial portion of said some scavenge port means and a portion of said exhaust port means is uncovered.
 7. The two cycle engine of claim 6 wherein said some scavenge port means terminates at a position spaced toward the distal end of said power chamber portion from said reference line, and a second scavenge port is positioned between said some scavenge port means and the reference line, said second scavenge port opening to a separate transfer passageway communicating with said scavenge pump whenever the piston uncovers said second scavenge port.
 8. In a two cycle engine having a piston with a piston skirt and a head and a rear side and being of the type which has a chamber comprising a scavenge pump system open to the rear side of the piston, a power cylinder having a bore defined by a cylinder wall and having a first end and a second distal end, said cylinder wall having a lower support portion, a piston slidably mounted in said cylinder for reciprocation through a piston stroke in said cylinder, said piston having a piston head timing edge movable through said piston stroke from a bottom dead center position adjacent a first end of said cylinder toward the second distal end of said cylinder, said cylinder having a diameter that is greater than 1.25 times the piston stroke, scavenge port means defined through the wall of said cylinder and communicating with said bore, exhaust port means defined through a wall of said cylinder and positioned adjacent the first end of said cylinder and extending toward the distal end a preselected amount, transfer passageway means connected to said scavenge port means and communicating with said scavenge pump system, said scavenge port means being positioned around the periphery of said cylinder wall to occupy a substantial portion of the cylinder wall, said scavenge port means in said cylinder positioned to have major portions thereof positioned closer to the first end of said cylinder than to said distal end of said cylinder, at least one of said scavenge port means having a distal portion, said distal portion being covered and uncovered by said piston as it reciprocates in said cylinder, valving means remote from said one scavenge port means to control communication between a transfer passageway connecTed to said one scavenge port means and said scavenge pump system to prevent communication between the scavenge port system and the cylinder through said one scavenge port means until the piston timing edge is substantially closer to its bottom dead center position than the distal edge of said one scavenge port means, and at least a portion of said exhaust port means is uncovered to permit pressure in said cylinder to reduce toward atmospheric pressure, said valving means comprising a scavenge pump system outlet port defined in the lower support portion of said cylinder wall and opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt open to the rear side of the piston and positioned to align with said outlet port during portions of the piston stroke.
 9. The combination as specified in claim 8 wherein said cylinder has a lower support portion, and said piston has a piston skirt supported by said lower portion during portions of the piston stroke and wherein said valving means comprises a scavenge pump system outlet port defined in the lower support portion and opening to said transfer passageway means and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke, and a control port defined in said piston skirt open to the rear side of the piston and positioned to align with said outlet port during portions of the piston stroke.
 10. The combination as specified in claim 8 wherein said distal edge of said scavenge port means extends at least 30 percent of the piston stroke from a reference line defined by the piston timing edge in bottom dead center position.
 11. The combination as specified in claim 8 wherein said one scavenge port means extends continuously from the distal edge thereof toward the first end of said cylinder substantially to a level corresponding to the bottom dead center position of the timing edge of said piston.
 12. The combination as specified in claim 8 wherein said scavenge port means includes first scavenge port means no greater than normal height and second extra height scavenge port means, said first scavenge port means opening to transfer passageways separate from the transfer passageway means for second extra height scavenge port means, said first scavenge port means being positioned between said second scavenge port means and said first end of the cylinder, and said valving means opening the transfer passageway means associated with the second scavenge port means to communication with the scavenge pump system during substantially the same portion of piston stroke as when the piston timing edge uncovers said first scavenge port means.
 13. In a two cycle engine having a cylinder and a power piston which has a skirt and a piston head timing edge, and which utilizes the underside of the power piston as a piston of a scavenge pump, said cylinder being defined by a cylindrical wall having an inner surface, scavenge port means defined through said wall and open to said cylinder, the cylinder having a lower wall portion and an upper portion, said scavenge port means opening to said upper portion, said upper portion having a first end adjacent a reference line along the piston timing edge with the piston in bottom dead center position and having a distal end, at least the major portion of said scavenge port means being located adjacent the first end of said upper portion of said cylinder, transfer passageway means leading from said scavenge pump to the scavenge port means, said scavenge port means being covered and uncovered by said piston as the piston reciprocates in the cylinder, said scavenge port means having a distal edge spaced from said reference line by a distance such that portions of said scavenge port means are uncovered by the piston timing edge prior to the time the pressure of combustion gases in the upper portion of the cYlinder has reduced to nearly rear compression pressure, said transfer passageway means opening to a scavenge pump outlet control port defined in the lower portion of said cylinder wall, said outlet control port aligning with and being blocked by the skirt of said piston when the piston is moving toward its bottom dead center position from the distal end to prevent amounts of said combustion gases harmful to engine performance from flowing back into the scavenge pump system, and a piston skirt control defined in said piston skirt and positioned in alignment with said outlet port only when the pressure of combustion gases has reduced to a desired level.
 14. In a two cycle engine having a piston with a piston skirt, a head end and an underside opposite the head end, said engine being of a type which has a chamber open to the underside of the piston comprising a scavenge pump system, a power cylinder defined by a wall and having a first end and a distal end, said piston being slidably mounted in said cylinder for reciprocation in said cylinder during a piston stroke from a bottom dead center position adjacent said first end to a distal position, said piston having a piston timing edge, an exhaust port defined in said cylinder on one side thereof, an opposite scavenge port means extending from a line generally along the piston head timing edge in bottom dead center position to a distal end at a level more than 30 percent of the piston stroke toward the distal end of the cylinder, with the major portion of said opposite scavenge port means being closer to the first end of said cylinder than the distal end, said opposite scavenge port means being positioned generally diametrically opposite the center of said exhaust port, a first transfer passageway connected to said opposite scavenge port means, side scavenge port means in the cylinder wall located circumferentially between the opposite scavenge port means and the exhaust port and being positioned closely adjacent the sides of the opposite scavenge port means and occupying a substantial portion of the cylinder wall circumference between the side edges of the opposite scavenge port means and the side edges of the exhaust port, second transfer passageway means opening to the scavenge pump system from said side scavenge port means, said scavenge port means being covered and uncovered by said piston timing edge as said piston reciprocates in said cylinder, and separate valving means timed by piston position to control communication between said first transfer passageway and said scavenge pump system to prevent such communication until the piston timing edge is closer to its bottom dead center position than the distal edge of said opposite scavenge port means, said separate valving means comprising a separate scavenge pump system outlet port opening to said first transfer passageway and to said scavenge pump system, said piston skirt blocking said outlet port during portions of the piston stroke when portions of the opposite scavenge port means of significant flow area and at least a portion of said exhaust port are uncovered by the piston, and a separate control port, corresponding to the outlet port, defined in said piston skirt and positioned to align with said outlet port to control communication between said first transfer passageway and said scavenge pump system through said outlet port.
 15. The combination as specified in claim 14 wherein said side scavenge port means includes at least two normal height scavenge port means, one on each side of said opposite scavenge port, at least two separate extra height scavenge port means, one on each side of said opposite scavenge port, said extra height scavenge port means being positioned toward the distal end of the cylinder from the normal height scavenge port means, said extra height scavenge port means having separate transfer passageways connected thereto, and piston skirt port valving means associated with the transfer passageways for the extra height port means to control comMunication between said scavenge pump system and the extra height scavenge port means, portions of said extra height scavenge port means being uncovered before the valving means associated therewith opens the extra height port means to the scavenge pump system. 